Bulk Band Gap and Surface State Conduction Observed in Voltage-Tuned Crystals of the Topological Insulator Bi$_2$Se$_3$

TL;DR

This study demonstrates voltage-tuned control of bulk and surface conduction in exfoliated Bi$_2$Se$_3$ topological insulator crystals, revealing a small bulk gap and surface state dominance at low temperatures, with implications for device applications.

Contribution

It provides experimental evidence of bulk and surface state conduction modulation in Bi$_2$Se$_3$ via electric field effect, highlighting the small bulk gap and surface state properties.

Findings

Bulk gap of approximately 50 meV identified

Surface states dominate low-temperature conduction

Gate voltage can change the sign of Hall density

Abstract

We report a transport study of exfoliated few monolayer crystals of topological insulator Bi$_2$Se$_3$ in an electric field effect (EFE) geometry. By doping the bulk crystals with Ca, we are able to fabricate devices with sufficiently low bulk carrier density to change the sign of the Hall density with the gate voltage $V_g$. We find that the temperature $T$ and magnetic field dependent transport properties in the vicinity of this $V_g$ can be explained by a bulk channel with activation gap of approximately 50 meV and a relatively high mobility metallic channel that dominates at low $T$. The conductance (approximately 2 $\times$ 7$e^2/h$), weak anti-localization, and metallic resistance-temperature profile of the latter lead us to identify it with the protected surface state. The relative smallness of the observed gap implies limitations for EFE topological insulator devices at room temperature.